1908 Tunguska explosion

We may finally know what happened at Tunguska...The explosion that came from nowhere7 July 2016 - Over 100 years after the most powerful explosion in documented history, researchers are still trying to figure out exactly what happened

On 30 June 1908, an explosion ripped through the air above a remote forest in Siberia, near the Podkamennaya Tunguska river. The fireball is believed to have been 50-100m wide. It depleted 2,000 sq km of the taiga forest in the area, flattening about 80 million trees. The earth trembled. Windows smashed in the nearest town over 35 miles (60km) away. Residents there even felt heat from the blast, and some were blown off their feet. Fortunately, the area in which this massive explosion occurred was sparsely inhabited. There were no official reports of human casualties, though one local deer herder reportedly died after he was thrust into a tree from the blast. Hundreds of reindeer were also reduced to charred carcasses.

The trees were still flattened 20 years after the explosion first occurred​

One eyewitness account said that "the sky was split in two, and high above the forest the whole northern part of the sky appeared covered with fire… "At that moment there was a bang in the sky and a mighty crash… The crash was followed by a noise like stones falling from the sky, or of guns firing." This "Tunguska event" remains the most powerful of its kind recorded in history – it produced about 185 times more energy than the Hiroshima atomic bomb (with some estimates coming in even higher). Seismic rumbles were even observed as far away as the UK. And yet, over a hundred years later researchers are still asking questions about what exactly took place on that fateful day. Many are convinced that it was an asteroid or a comet that was responsible for the blast. But very few traces of this large extraterrestrial object have ever been found, opening the way for more outlandish explanations for the explosion.

Comets are mostly made up of dust and ice​

The Tunguska region of Siberia is a remote place, with a dramatic climate. It has a long hostile winter and a very short summer, when the ground changes into a muddy uninhabitable swamp. This makes the area extremely hard to get to. When the explosion happened, nobody ventured to the site to investigate. This was partly because the Russian authorities had more pressing concerns than sating scientific curiosity, says Natalia Artemieva of the Planetary Science Institute in Tucson, Arizona. Political strife in the country was growing – World War One and the Russian Revolution were just a few years away. "There were only some publications in local papers, not even in St Petersburg or Moscow," she says.

100 years later the trees are still askew​

It was only a few decades later, in 1927, that a Russian team led by Leonid Kulik finally made a trip to the area. He had stumbled across a description of the event six years earlier and convinced Russian authorities that a trip would be worthwhile. When he got there, the damage was still immediately apparent, almost 20 years after the blast. He found a large area of flattened trees, spreading out about 31 miles (50km) wide in a strange butterfly shape. He proposed that an extraterrestrial meteor had exploded in the atmosphere. It puzzled him that there was no impact crater, or in fact, any meteoric remnants at all. To explain this, he suggested that the swampy ground was too soft to preserve whatever hit it and that any debris from the collision had been buried.

One reason for some of the more extreme explanations, was that higher than normal levels of radiation were detected. But it turns out that can be a byproduct of a highly energetic event like this. I don't remember the mechanism but radioactive byproducts can be produced from a non-nuclear explosion.

That led to suggestions ranging from a crashed UFO, to an alien nuclear weapon run amok.